Hexokinase
The first enzyme of the glycolytic pathway catalyses the transfer of the gamma-phosphoryl group of ATP to a hexose molecule. The trypanosome enzyme is exclusively associated with the glycosome and therefore the phosphorylation of glucose must take place either inside the glycosome or during the transport of the hexose into the organelle. It is not known whether hexokinase itself is involved in this transport or that there is a separate hexose transporter present in the glycosomal membrane. So far, no such transporter has been identified, although it was proposed by Opperdoes and Borst (1977). Hexokinase is active on glucose, mannose and fructose (Nwagwu and Opperdoes, 1982; Hara et al., 1997), for which it has similar affinities and on which it acts with similar rates. However, mannose 6-phosphate, that is produced inside the glycosome by the action of hexokinase is metabolised only very slowly, probably because of the low activity of mannose-6-phosphate isomerase in the glycosome (Misset and Opperdoes, unpublished), and thus accumulates inside the organelle, leading to an inhibition of glycolysis (Hara et al., 1997). Hexokinase is not inhibited by either glucose 6-phosphate or glucose 1,6-bisphosphate (Nwagwu and Opperdoes 1982; Opperdoes and Van Roy, unpublished), as are the hexokinases from most other sources, but it is selectively inhibited by a number glucose analogues (Trinquier et al., 1995; Willson and Opperdoes, unpublished). The T. brucei enzyme, contrary to most other hexokinases, is not specific for ATP; It also utilizes ITP, and to a lesser extent GTP, UTP and CTP (Misset and Opperdoes, 1984). Such specificity is probably not required for an enzyme that functions inside an organelle where ATP is supposed to be the major nucleotide. The enzyme has a molecular mass of 53 000 and an isoelectric point of 10. On a gel-filtration column it elutes as a hexameric protein (Misset and Opperdoes, 1984; Misset et al., 1986). It represents 0.25% of the total cellular protein and 6% of the glycosomal protein and it was calculated that some 180 molecules of hexokinase are present within one organelle (Misset et al., 1986). The gene encoding the enzyme has been cloned (Irrthum et al., unpublished). It codes for a polypeptide of 474 amino acids long, has a N-terminally located sequence that is reminiscent of a peroxisomal import signal, also called PTS-2, and has only 36 % positional identity with the human enzyme that has a molecular mass of 100 000. In view of the numerous differences with the mammalian enzyme, hexokinase proves to be an excellent target for drug design.